1. Field of the Invention
The present invention relates to a method for producing a binder resin having an excellent environmental stability with respect to the tribo electric charge and the image quality, and it also relates to a toner for electrophotography using such a binder resin.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, conventional electrophotography comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer and subsequently exposing the layer to eliminate the charge on the exposed portion and visualizing the formed image by adhering colored charged fine powder known as a toner to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
Therefore, a toner must meet the requirements not only in the development process but also in the transfer process and fixing process.
Generally, a toner undergoes mechanical frictional forces due to shear force and impact force during the mechanical operation in a developer device, thereby deteriorating after copying from several thousands to several ten thousands of sheets. Such deterioration of the toner can be prevented by using a tough resin having such a high molecular weight that it can withstand the above mechanical friction. However, this kind of a resin generally has such a high softening point that the resulting toner cannot be sufficiently fixed by a non-contact method such as oven fixing or radiant fixing with infrared rays, because of its poor thermal efficiency. Further, when the toner is fixed by a contact fixing method such as a heat roller fixing method, which is excellent in thermal efficiency and therefore widely used, it becomes necessary to raise the temperature of the heat roller in order to achieve sufficient fixing of the toner, which brings about such disadvantages as deterioration of the fixing device, curling of paper and an increase in energy consumption. Furthermore, the resin described above is poor in pulverizability, thereby remarkably lowering the production efficiency of the toner upon the production of the toner. Accordingly, the binder resin having too increased degree of polymerization and also too high softening point cannot be used.
Meanwhile, according to the heat roller fixing method, the surface of a heat roller contacts the surface of a visible image formed on an image-receiving sheet under pressure, so that the thermal efficiency is excellent and therefore widely used in various copying machines from high-speed ones to low-speed ones. However, when the surface of a heat roller contacts the surface of the visible image, the toner is likely to cause such a problem which is a so-called "offset phenomenon," wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper. In order to prevent this phenomenon, the surface of a heat roller may be coated with a material excellent in release properties, such as a fluororesin, or a releasing agent such as a silicone oil may be applied on the surface of a heat roller. However, the method of applying a silicone oil, necessitates a larger-scale fixing device, which is not only expensive but also complicated, which in turn may undesirably bring various problems.
Processes for improving the offset phenomenon by unsymmetrizing or crosslinking the resins have been known (see Japanese Patent Examined Publication No. 57-493 and Japanese Patent Laid-Open Nos. 50-44836 and 57-37353), but the fixing temperature has not yet been improved by these processes.
Since the lowest fixing temperature of a toner is generally between the temperature of low-temperature offsetting of the toner and the temperature of the high-temperature offsetting thereof, the serviceable temperature range of the toner is from the lowest fixing temperature to the temperature for high-temperature offsetting. Accordingly, by lowering the lowest fixing temperature as much as possible and raising the temperature causing high-temperature offsetting as much as possible, the serviceable fixing temperature can be lowered and the serviceable temperature range can be widened, which enables energy saving, high-speed fixing and prevention of curling of paper.
From the above reasons, the development of a binder resin and a toner excellent in fixing ability and offset resistance have always been desired.
In order to meet the requirements described above, in cases where styrene resins are used as binder resins, methods in which paraffin waxes, low-molecular weight polyolefins and the like are added as offset inhibitors have been known (see Japanese Patent Laid-Open No. 49-65232, 50-28840 and 50-81342). In these references, however, such problems arise that when the amount of the offset inhibitors added are small, sufficient effects cannot be achieved by the addition thereof, and that when it is large, the deterioration of the obtained developers is undesirably rapid.
The polyester resins have inherently good fixing ability, and as disclosed in U.S. Pat. No. 3,590,000, the toner using it can be sufficiently fixed even by a non-contact type fixing method. However, since the offset phenomenon is likely to take place, it has been difficult to use these polyester resins in the heat roller fixing method. Attempts using polycarboxylic acids have been made to improve the offset resistance in the polyester resins (see Japanese Patent Laid-Open Nos. 50-44836, 57-37353 and 57-109875). However, in these methods, a sufficiently good offset resistance to a practical level cannot be achieved, and even if such a good offset resistance is achieved, the low-temperature fixing ability inherently owned by the polyester resins is in turn deteriorated, and the pulverizability of the resin itself and that of the mixed material in the toner production become poor.
In order to achieve a high resolution in the electrophotographic method, attempts have been made to make the particle diameter of the toners small. However, there have been some problems in finding out how to surely obtain toners with a small particle diameter at a low cost.
Also, in the case where the polyester resin has a certain acid value and a certain hydroxyl value so as to particularly provide good dispersion of the coloring agent, much restriction is placed in toner compositions when compared with the case where the styrene-acrylic resin is used in order to maintain a good environmental stability in the tribo electric charge and the image quality after the production of toners.
In order to solve the above problems, the following methods for blending polyester resins with styrene-acrylic resins have been known. For instance, examples of such methods include:
(1) Methods for blending polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open Nos. 49-6931, 54-114245, 57-70523, and 2-161464); PA1 (2) Methods for chemically binding polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open No. 56-116043); PA1 (3) Methods for copolymerizing unsaturated polyesters with vinyl monomers (see Japanese Patent Laid-Open Nos. 57-60339, 63-279265, 1-156759 and 2-5073); PA1 (4) Methods for copolymerizing polyester resins having an (meth)acryloyl group with vinyl monomers (see Japanese Patent Laid-Open No. 59-45453); PA1 (5) Methods for copolymerizing reactive polyesters with vinyl monomers in the presence of polyester resins (see Japanese Patent Laid-Open No. 2-29664); and PA1 (6) Methods for forming a block copolymer by binding polyester resins and vinyl resins with an ester bond (see Japanese Patent Laid-Open No. 2-881). PA1 (1) A method of producing a binder resin comprising the steps of blending in advance (a) a starting material monomer mixture for two polymerization reaction types, (b) a compound which can react with both of the starting material monomers for the two polymerization reaction types, and (c) a tricarboxylic or higher polycarboxylic acid or a derivative thereof, said two polymerization reaction types having independent reaction paths to each other in one reaction vessel; and concurrently carrying out the two polymerization reactions in one reaction vessel; and PA1 (2) A toner for electrophotography using the binder resin produced by the method mentioned above.
However, since the polyester resins have inherently poor compatibility with the styrene-acrylic resins, mere mechanical blending of the components may result in causing such troubles as background in the formed images of the toner produced in certain blending ratios. Further, in the case where the vinyl monomers are copolymerized with the reactive polyesters, it is applicable only in a restricted compositional range in order not to allow gelation to take place.
Also, a method in which a starting monomer mixture for two polymerization reaction types are blended in advance and the two polymerization reactions are concurrently carried out has been proposed (see Japanese Patent Laid-Open No. 4-142301). Although the binders for toner production disclosed herein have an islands-sea structure formed by dispersing a polyester resin in a styrene-acrylic resin, the diameter of the dispersed particles are larger than 2 .mu.m. Therefore, in this reference, although the fixing temperature can be lowered, a further improvement in a life of the toner cannot be sufficiently obtained.
Accordingly, there is an increasing demand for a binder for electrophotography which is excellent in the low-temperature fixing ability and the offset resistance, has an environmental stability in a tribo electric charge and an image quality, which provides an excellent durability in the resulting toner.